well, there should be different "ranks" of zombies. slow, shuffling ones are the newly formed zombies, medium speed ones are people who have been zombies for a while, while the highest "rank" would be well armored and have guns. in a skirmish, the new zombies can morph into the better ones, etc.
the Bio-Chemist unit needs a gas mask, and this would give him an immunity to other bio attacks (or a heightened resistance). and they need morph forms so they can be trained to become more specialized. e.g. the Necro-Chemist can turn dudes into zombies, while the Bio-Terrorist has many (ironically) barely lethal attacks (strength down, gradual HP loss, etc).
their tanks need to be chemical spraying machines. the anti-vehicle tanks would spray a corrosive material that does nearly nothing to infantry, then a anti-infantry vehicle that does the opposite.